The present invention relates to techniques for inhibiting the polymerization of a monomer such as a conjugated diene, aromatic vinyl, ethylenically unsaturated nitrile or xcex1-olefin, and more particularly to a polymerization-inhibiting composition, a polymerization inhibitor and a polymerization-inhibiting method for inhibiting the occurrence of unfavorable premature polymerization in various operating steps such as the production, purification, storage, shipment, preparation and use of such a monomer, a mixture of the monomers or a hydrocarbon mixture containing the monomer.
A monomer such as a conjugated diene, aromatic vinyl, ethylenically unsaturated nitrile or xcex1-olefin is easy to cause premature polymerization in various operating steps such as the production, purification, storage, shipment, preparation and use thereof. The occurrence of the premature polymerization causes contamination of the monomer with a polymer, increase of viscosity, gelling, loss in reactivity, etc. In addition, by the premature polymerization, a heat exchanger, storage container, transfer line, pump, distilling apparatus or the like is stained with the polymer, and so problems such as increase in the cost of washing, lowering in production efficiency and loss of material occur. The problems relating to such premature polymerization will be described more specifically taking the case of a purification step of a conjugated diene.
Conjugated dienes, conjugated diene-containing hydrocarbon mixtures (for example, C4 hydrocarbon fraction and C5 hydrocarbon fraction), conjugated diene-containing unsaturated olefin hydrocarbon mixtures (mixtures in a recovery step of olefin hydrocarbon compounds after gas and liquid cracking or decomposing processes), conjugated diene-containing monomer mixtures (for example, monomer mixtures for production of synthetic rubbers) and the like are easy to undergo polymerization of the conjugated dienes and/or copolymerization of the conjugated dienes with any other copolymerizable unsaturated compound upon distillation, extractive distillation, extraction, countercurrent extraction, hydrogenation or hydrotreating, hydrorefining, heat treatment, other similar treatments, preheating before treatment, storage, transfer or processing.
For example, when a purified conjugated diene is isolated and recovered from a conjugated diene-containing hydrocarbon mixture by a distillation process including extractive distillation, a polymerization reaction is easy to occur to form a solvent-soluble polymer or a crosslinked, solvent-insoluble polymer. The solvent-soluble polymer is called a rubbery polymer and stains various apparatus or devices such as an extractive distillation column, distillation column, heat exchanger and piping. The crosslinked, solvent-insoluble polymer is a porous insoluble polymer and is called a popcorn polymer due to its appearance. This popcorn polymer is particularly undesirable because not only it self-multiplies in the presence of the vapor or liquid of the conjugated diene to rapidly clog the apparatus, but it is extremely difficult to remove and control. Once the popcorn polymer is formed, it multiplies, so to speak, exponentially, in that it serves as a seed. Since the popcorn polymer is a strong crosslinked polymer, it is insoluble in any already-known solvent and not melted. Accordingly, in order to remove the popcorn polymer, there is no effective cleaning method, but it is cleaned out by a mechanical means. The cleaning of the apparatus requires to suspend and disjoint it so as to mechanically remove the deposit of the polymer, and so it takes time, and economical disadvantage is unavoidable. In addition, since the popcorn polymer cannot be completely removed by the mechanical cleaning, the popcorn polymer remaining in the apparatus in a trace amount serves as a seed to start the multiplication of the popcorn polymer again when the operation of the apparatus is resumed.
In a process of preparing a purified conjugated diene by subjecting a hydrocarbon mixture containing the conjugated diene to a distillation process including extractive distillation, conditions liable to induce a polymerization reaction, such as coexistence of a gas phase with a liquid phase, moderate operating temperature, high monomer purity, mixing of water and presence of iron rust, gather. Accordingly, there have heretofore been proposed methods making use of various kinds of polymerization inhibitors. However, in some cases, the insufficient polymerization inhibiting effects thereof may have made it difficult to prevent the formation of a rubbery polymer and/or a popcorn polymer, so that the apparatus is clogged. When the polymerization inhibitor is used in a large amount to enhance the polymerization inhibiting effect, there occur such problems that a tar-like product is formed to waste energy, and the extraction efficiency of the extractive distillation is lowered.
In a process of recovering an olefin hydrocarbon compound such as ethylene, propylene, butene, butadiene or a mixture thereof after gas and liquid cracking or decomposing processes, treatments such as an isolating process of various kinds of olefin hydrocarbon compounds are conducted by conversion by hydrogenation, distillation or extraction of olefin compounds and acetylene compounds. Deposit (scale) considered to be attributable to the polymerization of conjugated dienes and/or the like is easy to form on apparatus for these treatments. When such deposit is built up to an excessive extent, thermal efficiency of the apparatus and isolation efficiency of a distillation column are lowered, and clogging of piping is caused. In addition, a monomer mixture containing a conjugated diene and a vinyl aromatic compound such as styrene has been known to show a tendency to polymerize during its storage.
There has heretofore been proposed a method of distilling a C5 hydrocarbon fraction in the presence of N,N-dialkylhydroxylamine in order to inhibit the polymerization of a conjugated diene-containing petroleum fraction in a distillation apparatus (Japanese Patent Application Laid-Open No. 112304/1975). However, the mere use of N,N-dialkylhydroxylamine is not sufficient in the polymerization-inhibiting effect. U.S. Pat. No. 3,371,124 has proposed a method of using N,N-dialkyl-hydroxylamine and the oxalate [bis(diethylhydroxylamine) oxalate] thereof as polymerization inhibitors in order to inhibit the formation of a popcorn polymer in a recovery system by fractional distillation of a monomer containing at least one conjugated diene discharged from a production process of SBR. The oxalate can be obtained by reacting N,N-dialkylhydroxylamine with oxalic acid. However, oxalic acid involves a problem that it corrodes an extractive distillation column. The mere use of the oxalate is not sufficient in the polymerization-inhibiting effect.
Japanese Patent Application Laid-Open No. 189810/1992 discloses a method of causing [Group A] hydroquinone, hydroquinone monomethyl ether, p-methoxyphenyl, phenothiazine, piperidine, etc. and [Group B] phosphorus-containing compounds such as phosphoric acid and potassium phosphate to coexist with a molecular oxygen-containing gas in order to inhibit thermal polymerization upon epoxidation of a double bond in a cyclohexenyl ring by causing an epoxidizing agent to act on a mixture of (meth)acrylates having a cyclohexenyl group in their ester moieties. However, it has been found that when a combination of hydroquinone shown as a representative example of Group A in this publication with a phosphorus-containing compound in Group B is used to conduct extractive distillation of a conjugated diene-containing hydrocarbon mixture, a sufficient polymerization-inhibiting effect cannot be achieved.
Further, it has heretofore been proposed to use a compound having a stable NO radical (free radical) in its molecule or a compound forming a stable NO radical in situ under treating conditions as a polymerization inhibitor (Japanese Patent Publication No. 26639/1992, etc.). However, the mere use of these compounds fails to achieve a sufficient polymerization-inhibiting effect.
Such problems of premature polymerization are easy to occur at various operating steps in not only conjugated dienes, but also many other monomers.
It is an object of the present invention to provide a polymerization inhibitor and a method of inhibiting polymerization for inhibiting the occurrence of unfavorable premature polymerization in various operating steps such as the production, purification, storage, shipment, preparation and use of a monomer such as a conjugated diene, aromatic vinyl, ethylenically unsaturated nitrile or xcex1-olefin, a mixture of the monomers or a hydrocarbon mixture containing such a monomer.
Another object of the present invention is to provide a polymerization-inhibiting composition containing the monomer and polymerization inhibitor described above.
A particular object of the present invention is to provide a novel polymerization inhibitor, method of inhibiting polymerization and polymerization-inhibiting composition containing a conjugated diene and the polymerization inhibitor for inhibiting (co)polymerization of the conjugated diene upon the treatment, storage or the like of the conjugated diene, a conjugated diene-containing hydrocarbon mixture, a conjugated diene-containing unsaturated olefin hydrocarbon mixture, a conjugated diene-containing monomer mixture, or the like at a high level.
The present inventors have carried out an extensive investigation with a view toward overcoming the above-described problems involved in the prior art. As a result, it has been found that the combined use of at least one compound selected from the group consisting of a compound having an NO radical (NO) in its molecule and a precursor compound capable of forming an NO radical and a phosphorus-containing compound as a polymerization inhibitor is extremely effective for the inhibition of premature polymerization of various kinds of monomers such as conjugated dienes.
The polymerization inhibitor according to the present invention can markedly inhibit the formation of a popcorn polymer and a rubbery polymer when it is caused to exist in a distillation step in a process for isolating and producing a purified conjugated diene by conducting a distillation process including extractive distillation from, for example, a conjugated diene-containing hydrocarbon mixture. The polymerization inhibitor according to the present invention is extremely effective for not only hydrocarbon mixtures and the like containing a conjugated diene in a great amount, but also hydrocarbon mixtures and the like containing a conjugated diene in a small amount. The polymerization inhibitor according to the present invention is also effective for the inhibition of polymerization of monomers such as aromatic vinyls, ethylenically unsaturated nitrites and xcex1-olefins. The present invention has been led to completion on the basis of these findings.
According to the present invention, there is thus provided a polymerization-inhibiting composition comprising at least one compound (a) selected from the group consisting of a compound having an NO radical in its molecule and a precursor compound capable of forming an NO radical, a phosphorus-containing compound (b), and at least one monomer (c) selected from the group consisting of a conjugated diene, an aromatic vinyl, an ethylenically unsaturated nitrile and an xcex1-olefin, wherein a weight ratio of the compound (a) to the phosphorus-containing compound (b) is 1:10 to 100:1.
According to the present invention, there is also provided a polymerization inhibitor for at least one monomer (c) selected from the group consisting of a conjugated diene, an aromatic vinyl, an ethylenically unsaturated nitrile and an xcex1-olefin, comprising at least one compound (a) selected from the group consisting of a compound having an NO radical in its molecule and a precursor compound capable of forming an NO radical, and a phosphorus-containing compound (b), wherein a weight ratio of the compound (a) to the phosphorus-containing compound (b) is 1:10 to 100:1.
According to the present invention, there is further provided a method for inhibiting polymerization, which comprises causing at least one compound (a) selected from the group consisting of a compound having an NO radical in its molecule and a precursor compound capable of forming an NO radical, and a phosphorus-containing compound (b) to coexist at a weight ratio of the compound (a) to the phosphorus-containing compound (b) of 1:10 to 100:1 with at least one monomer (c) selected from the group consisting of a conjugated diene, an aromatic vinyl, an ethylenically unsaturated nitrile and an xcex1-olefin.